Plastics resins include a wide variety of resins, for example, general-purpose resins such as polyolefin resins, polyvinyl chloride resins, ABS resins, polystyrene resins and methacrylic resins, engineering plastics resins such as polycarbonate resins, polyethylene terephthalate resins and polyamide resins, and the like. They are molded into sheets, films, laminates, cast articles, etc. according to their characteristics and used in various fields. Many of the compositions using these plastics resins are required to have hiding properties, and usually contain titanium dioxide pigments high in refractive index for visible light.
However, the surface of titanium dioxide pigments has a high catalytic activity, and plastics resin compositions containing them are low in light resistance and weather resistance and are apt to show deterioration or discoloration with lapse of time. Furthermore, when titanium dioxide pigments are blended in resin compositions and the compositions are processed at high temperatures, surface imperfections generally called lacing (bubbling) or pinholes sometimes occur, which seriously damage the commercial value of the resin compositions. Such surface imperfections are considered to be caused by water volatilized from titanium dioxide pigments.
Among plastics resins, engineering plastics resins have features of excellent impact resistance, heat resistance, electrical characteristics and transparency and high working accuracy, but many of them are high in melting temperature and must be worked at temperatures higher than the working temperatures for general-purpose resins, sometimes resulting in surface imperfections mentioned above. In the case of engineering plastics resins, the surface imperfections are apt to appear as white streaky patterns on the surface of injection molded articles, which are sometimes called silver streaks. The engineering plastics resins have the further problem that hydrolysis reaction of the resins takes place due to the volatilized water, which causes reduction in molecular weight of the resins to damage the strength. There is another problem that some kinds of resins such as polycarbonate resins are apt to discolor to yellowish color tone at high temperatures.
In order to avoid occurrence of such phenomena, for example, it is known to coat a hydrolyzate of an organic silane compound on the surface of a titanium dioxide pigment used for general-purpose plastics resin compositions, particularly, polyolefin resin compositions (Patent Document 1). According to this technology, the titanium dioxide pigment is made hydrophobic by coating with the hydrolyzate to inhibit adsorption of water, thereby reducing the amount of water volatilized and inhibiting formation of surface imperfections.
Furthermore, as for the titanium dioxide pigment to be blended in polycarbonate resin compositions and polyethylene terephthalate resin compositions, it is known to coat the titanium dioxide pigment with a hydrous oxide such as hydrous silica or hydrous alumina, then with a reactive polysiloxane and furthermore with an organometallic compound such as an organosilicon compound (Patent Document 2).
Patent Document 1: JP-A-10-324817
Patent Document 2: JP-A-9-3211
The technology of Patent Document 1 gives a high effect to inhibit occurrence of surface imperfections, but is insufficient in reducing the catalytic activity of titanium dioxide pigment to improve light resistance and weather resistance.
The titanium dioxide pigment disclosed in Patent Document 2 shows excellent effect to inhibit hydrolysis or to give thermal discoloration resistance of engineering plastics resin composition, but is insufficient in inhibiting formation of silver streaks.